Principles of Color Flow Mapping

Notes

Cards

• basicI.J-001
  Is color Doppler based on pulsed or continuous ultrasound?
  → Pulsed ultrasound. Consequently, color Doppler has range resolution and is subject to aliasing.
• basicI.J-002
  What velocity does a color Doppler pixel represent — peak or mean?
  → Mean velocity of the packet of pulses at that location. Spectral Doppler reports peak; color reports mean.
• clozeI.J-003
  A color Doppler 'packet' or 'ensemble length' is typically ~8 pulses per scan line.
• basicI.J-004
  What is the trade-off with a larger color Doppler packet size?
  → More accurate velocity estimates and better low-velocity sensitivity, but lower frame rate (worse temporal resolution).
• basicI.J-005
  How do you distinguish true flow reversal from aliasing on color Doppler?
  → Aliasing wraps through the extremes of the color map (red → yellow → cyan → blue). True flow reversal transitions through the center of the map (through black), because velocity truly passes through zero.
• basicI.J-006
  How does shifting the baseline affect the color Doppler aliasing limit?
  → Shifting the baseline lets you display velocities up to ~2× the original Nyquist limit in the direction of shift, at the cost of the opposite direction's range.
• basicI.J-007
  How does narrowing the color sector width improve the color image?
  → Fewer sampling lines → higher frame rate (better temporal resolution) and/or denser velocity data at the same frame rate.
• basicI.J-008
  What determines the PRF (and thus Nyquist limit) of color Doppler?
  → The maximum sector depth. Deeper sector → longer PRP → lower PRF → lower Nyquist limit → more aliasing.
• basicI.J-009
  Should color Doppler angle be optimized for perpendicular or parallel to flow?
  → Parallel (0° or 180°) — same principle as spectral Doppler. Perpendicular flow shows no color (cos 90° = 0).